1. Field of the Invention
The present invention relates to a method for manufacturing an encapsulated probe or measuring head on sheathed thermocouples, in which thermocouple wires are embedded in an outer metal sheath with the interposition of an insulating material, and wherein a thermal bead is produced by either an electric or autogeneous process at the sections of the thermocouple wires projecting from the metal sheath at the measuring head end of the wire.
2. Discussion of the Prior Art
Known in the present state of the art are sheathed thermocouples with open and closed probe or measuring heads. In the first mentioned construction, the metal sheath reaches to a point shortly ahead of the measuring location, however, the thermocouple bead is freely welded, soldered, cemented or adhered to the support therefor. The sealing of the hygroscopic insulator is effected through adhesive materials based on epoxide polyamide or silicon resin. When the permissible boundary temperatures (mostly between 300.degree. C. and 350.degree. C.) are exceeded, this protection is eliminated. The result will be erroneous readings up to the complete failure of the signal pickup. Furthermore, there is no protection or only limited degree of protection against aggressive media. This protection is afforded only to thermocouples in which the tip is welded and the thermal bead form an integral part of the measuring probe. However, they also possess, in their construction with an oval cross-section, only a linear zone of contact with the substratum. Hereby, the bead is separated from the actual test location by half the diameter of the tube or by the minor semi-axis of the ellipse. At measurements with large temperature gradients in the boundary layer near the wall or at high flow velocities there will appear at the sheath of the thermocouple itself errors in the output signals. In addition to the foregoing, these commonly marketed sheathed thermocouples can be joined directly with the substratum through spot welding only with considerable difficulties and serious risks to the measuring probe element.